Can we please stop these Quantum Computer spams going around recently all over the forum? There are numerous topics regarding "Can bitcoin be destroyed using QC?" and people making the same posts over and over again. As of now, the Quantum computers would prove a threat when people tend to reuse the same addresses over and over again for receiving the coins. On such instances, public key of the bitcoin address might be figured out which with the help of QC can be helpful in displaying the private keys.

Currently there are no real ways or ideas where a QC can break through the SHA256 function and find out a public key but OTOH a future QC can break a public key cryptography and reveal the private key. The QC pose a threat for PKC type of algorithms but they cannot break the hash functions and if they come nearer to them, we can certainly double up the bit sizes. QC require around 1500 qubits to crack the ECDSA algorithm and the current working IBM Q System One has only 20 quantum bit. So it might take around 10-20 years for a quantum computer to reach at least 1000 quantum bits.But at least our bitcoin developers would find a way to adopt Bitcoin with Post-Quantum Cryptography techniques.

The recent Google's Quantum Supremacy is said to have an array of only 54 qbits which is only 2.7 times the qbits developed by IBM Q. When we find out a possible way to break ECDSA, we would be adopting for the post-quantum cryptography. Security Engineers and Hackers are always same and neither of them can win the game of security always. For instance, if a hacker exploits a system the security expert finds out a way to minimize the vulnerability at the earliest.

Everyone need to have a deep read on this topic : I don't believe Quantum Computing will ever threaten Bitcoin before starting any topics further on quantum computers and their threat to Bitcoin or cryptography.